v0.12.1 — packaging fixes

Upgrade from 0.12.0 if you load dendra with require() or use TypeScript with
CommonJS. No API changes.

• CommonJS shares one copy of core. Each require() subpath
  (dendra/next, dendra/adapter, …) used to bundle its own copy, so
  instanceof DendraError failed across them, orPage missed core's
  NOT_FOUND, and .after() hooks reported a guard's error as INTERNAL.
• CommonJS types. TypeScript projects compiling to CommonJS got the ESM
  types for every entry; they now get .d.cts files. Older
  moduleResolution: "node" setups resolve the subpaths too.
• dendra/package.json is exported, and engines declares Node 18 or later.
• followJob and useJob retry as soon as the browser comes back online,
  instead of waiting out the backoff.
• The package is checked with publint and are-the-types-wrong in CI, and the
  example app is built and tested against each commit.